<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Limitations and Configuration</title>
<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="../boost_pfr.html" title="Chapter 28. Boost.PFR 2.0">
<link rel="prev" href="tutorial.html" title="Tutorial">
<link rel="next" href="how_it_works.html" title="How it works">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td>
<td align="center"><a href="../../../index.html">Home</a></td>
<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="tutorial.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_pfr.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="how_it_works.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_pfr.limitations_and_configuration"></a><a class="link" href="limitations_and_configuration.html" title="Limitations and Configuration">Limitations and
    Configuration</a>
</h2></div></div></div>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
        Recommended C++ Standards are C++17 and above. Library requires at least
        C++14! Pre C++14 compilers (C++11, C++03...) are not supported.
      </p></td></tr>
</table></div>
<p>
      Boost.PFR library works with types that satisfy the requirements of <code class="computeroutput"><span class="identifier">SimpleAggregate</span></code>: aggregate types without
      base classes, <code class="computeroutput"><span class="keyword">const</span></code> fields, references,
      or C arrays:
    </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">simple_aggregate</span> <span class="special">{</span>  <span class="comment">// SimpleAggregate</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="identifier">age</span><span class="special">;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uuids</span><span class="special">::</span><span class="identifier">uuid</span> <span class="identifier">uuid</span><span class="special">;</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">empty</span> <span class="special">{</span>             <span class="comment">// SimpleAggregate</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">aggregate</span> <span class="special">:</span> <span class="identifier">empty</span> <span class="special">{</span> <span class="comment">// not a SimpleAggregate</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="identifier">age</span><span class="special">;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uuids</span><span class="special">::</span><span class="identifier">uuid</span> <span class="identifier">uuid</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
      The library may work with aggregates that don't satisfy the requirements of
      <code class="computeroutput"><span class="identifier">SimpleAggregate</span></code>, but the behavior
      tends to be non-portable.
    </p>
<h3>
<a name="boost_pfr.limitations_and_configuration.h0"></a>
      <span class="phrase"><a name="boost_pfr.limitations_and_configuration.configuration_macro"></a></span><a class="link" href="limitations_and_configuration.html#boost_pfr.limitations_and_configuration.configuration_macro">Configuration
      Macro</a>
    </h3>
<p>
      By default Boost.PFR <span class="bold"><strong>auto-detects your compiler abilities</strong></span>
      and automatically defines the configuration macro into appropriate values.
      If you wish to override that behavior, just define:
    </p>
<div class="table">
<a name="boost_pfr.limitations_and_configuration.linkmacro"></a><p class="title"><b>Table 28.2. Macros</b></p>
<div class="table-contents"><table class="table" summary="Macros">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
              <p>
                Macro name
              </p>
            </th>
<th>
              <p>
                Effect
              </p>
            </th>
</tr></thead>
<tbody>
<tr>
<td>
              <p>
                <span class="bold"><strong>BOOST_PFR_USE_CPP17</strong></span>
              </p>
            </td>
<td>
              <p>
                Define to <code class="computeroutput"><span class="number">1</span></code> if you wish
                to override Boost.PFR choice and use C++17 structured bindings for
                reflection. Define to <code class="computeroutput"><span class="number">0</span></code>
                to override Boost.PFR choice and disable C++17 structured bindings
                usage.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <span class="bold"><strong>BOOST_PFR_USE_LOOPHOLE</strong></span>
              </p>
            </td>
<td>
              <p>
                Define to <code class="computeroutput"><span class="number">1</span></code> if you wish
                to override Boost.PFR choice and exploit <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2118" target="_top">CWG
                2118</a> for reflection. Define to <code class="computeroutput"><span class="number">0</span></code>
                to override Boost.PFR choice and disable CWG 2118 usage.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <span class="bold"><strong>BOOST_PFR_USE_STD_MAKE_INTEGRAL_SEQUENCE</strong></span>
              </p>
            </td>
<td>
              <p>
                Define to <code class="computeroutput"><span class="number">0</span></code> if you are
                hit by the template instantiation depth issues with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_integer_sequence</span></code> and wish
                to use Boost.PFR version of that metafunction. Define to <code class="computeroutput"><span class="number">1</span></code> to override Boost.PFR detection logic.
              </p>
            </td>
</tr>
<tr>
<td>
              <p>
                <span class="bold"><strong>BOOST_PFR_HAS_GUARANTEED_COPY_ELISION</strong></span>
              </p>
            </td>
<td>
              <p>
                Define to <code class="computeroutput"><span class="number">0</span></code> if your compiler
                does not implement C++17 guaranteed copy elision properly and fails
                to reflect aggregates with non-movable fields. Define to <code class="computeroutput"><span class="number">1</span></code> to override Boost.PFR detection logic.
              </p>
            </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h3>
<a name="boost_pfr.limitations_and_configuration.h1"></a>
      <span class="phrase"><a name="boost_pfr.limitations_and_configuration.details_on_limitations"></a></span><a class="link" href="limitations_and_configuration.html#boost_pfr.limitations_and_configuration.details_on_limitations">Details
      on Limitations</a>
    </h3>
<p>
      The Boost.PFRs reflection has some limitations that depend on a C++ Standard
      and compiler capabilities:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Static variables are ignored
        </li>
<li class="listitem">
          T must be aggregate initializable without empty base classes
        </li>
<li class="listitem">
          if T contains C arrays or it is inherited from non-empty type then the
          result of reflection may differ depending on the C++ version and library
          configuration
        </li>
<li class="listitem">
          Additional limitations if <span class="bold"><strong>BOOST_PFR_USE_CPP17 ==
          0</strong></span>:
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
                Non of the member fields should have a template constructor from
                one parameter.
              </li>
<li class="listitem">
                Additional limitations if <span class="bold"><strong>BOOST_PFR_USE_LOOPHOLE
                == 0</strong></span>:
                <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: square; ">
<li class="listitem">
                      T must be constexpr aggregate initializable and all its fields
                      must be constexpr default constructible
                    </li>
<li class="listitem">
                      <code class="computeroutput"><a class="link" href="../boost/pfr/get.html" title="Function get">boost::pfr::get</a></code>,
                      <code class="computeroutput"><a class="link" href="../boost/pfr/structure_to_tuple.html" title="Function template structure_to_tuple">boost::pfr::structure_to_tuple</a></code>,
                      <code class="computeroutput"><a class="link" href="../boost/pfr/structure_tie.html" title="Function structure_tie">boost::pfr::structure_tie</a></code>,
                      <code class="computeroutput"><a class="link" href="../reference_section.html#header.boost.pfr.core_hpp" title="Header &lt;boost/pfr/core.hpp&gt;">boost::pfr::tuple_element</a></code>
                      require T to be a POD type with built-in types only.
                    </li>
</ul></div>
              </li>
</ul></div>
        </li>
</ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2016-2021 Antony Polukhin<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="tutorial.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_pfr.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="how_it_works.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
